The present invention relates to a voltage reference circuit with linearized temperature behavior.
As is known, the voltage reference is an essential block of integrated circuits. Said block can comprise configurations using Zener diodes or a so-called band-gap structure, a typical configuration whereof is shown in FIG. 1. Said illustrated structure is currently preferred to configurations using Zener diodes, since it has some advantages, among which the low value of its output voltage, typically 1.2 V, which allows to extend its compatibility with power supply sources, and good thermal stability.
With reference to the diagram of FIG. 1, in particular to the transistors Q.sub.1 and Q.sub.2, simple calculations show that ##EQU1## where A is the ratio between the emitter areas of Q.sub.1 and Q.sub.2 ; I.sub.S is the converse saturation current, ##EQU2## is a corrective parameter which is related to the employed technology and is independent from the temperature.
By derivation with respect to the temperature, the following is obtained: ##EQU3##
By analyzing this last equation, it has been seen that .multidot..differential..DELTA.V .sub.BE /.differential.T is constant and positive, and therefore the primitive function has a rising linear behavior; while .multidot..differential.V.sub.BE /.differential.T is not constant and is negative, and therefore the voltage V.sub.BE (T) has a non-linear decreasing behavior. This situation is exemplified in FIGS. 2a and 2b, which respectively illustrate the derivative of the voltage drop on R.sub.2 (directly proportional to the derivative of .DELTA.V.sub.BE with respect to the temperature) and the derivative of the base-emitter drop with respect to the temperature.
In a significant temperature range (typical for applications in the motor-vehicle field) comprised between -40.degree. C. and 150.degree. C., three different situations are possible, namely:
if .differential.V.sub.BE /.differential.T&gt;.differential..alpha..DELTA.V.sub.BE /.differential.T in absolute value in the entire range being considered, the voltage V.sub.REF (T) will have an always decreasing behavior;
if instead .differential.V.sub.BE /.differential.T&lt;.differential..alpha..DELTA.V.sub.BE /.differential.T (always in absolute value in the entire range), V.sub.REF (T) will always have a rising behavior;
if, always within the initially considered range, the second of the two described conditions is true initially and the first one is subsequently true, the derivative of the voltage V.sub.REF (T) with respect to the temperature will be initially positive and subsequently negative (see FIG. 2c) and the primitive function will have a parabolic plot.
More generally, it can be said that the voltage V.sub.REF has a parabolic behavior in which the position of the maximum value can be internal or external to the temperature range being considered. With a same voltage V.sub.BE, the position of this point is linked to the voltage V.sub.REF to be obtained at a given reference temperature (environmental temperature is usually considered). This reference voltage value therefore determines the value of the resistor R.sub.2.
These conclusions are illustrated in FIGS. 2a, 2b, 2c and 3, in which three different values of the resistor R.sub.2 have been assumed and therefore three different plots have been obtained. In particular, the curves 1, 2 and 3 relate to decreasing values of the resistor R.sub.2 which entail a shift of the sign-change point of the curve .differential.V.sub.REF /.differential.T, i.e. a variation in the slop-change point of the primitive, which will therefore have one of the three behaviors shown in FIG. 3. This behavior is in any case merely theoretical, as it is determined by solving a mathematical equation; in practive, however, the unavoidable process spreads make such a behavior unattainable.